We will investigate the biotransformation of orally administered methylmercury to inorganic mercury in the California sea lion, Zalophus californianus. In vitro studies of various tissues from sea lions and other marine mammals will be made to describe the bio-pathway by which such detoxification occurs. This demethylation process may be the reason that these species are able to cope with a high natural intake of methylmercury, which is concentrated in their prey species. Differences in the tissue distribution of inorganic and organic mercury in the sea lion suggest that these may be associated with a metal-binding protein, such as metallothionein, which may be active in the storage and/or transport of mercury within the animal. Isolation and characterization of mercury-binding proteins will be carried out in an effort to determine their physiological significance.